Recently conventional mechanical drive systems are changed to electric drive systems using motors in vehicles to meet requirements for saving spaces, improving assembling work, improving controllability and the like purposes.
One example of such electric drive systems is a system, which drives a range switchover mechanism of an automatic transmission of a vehicle by a motor. In this system, when a selected range is changed by a shift operation or the like made by a driver, the motor is rotationally driven to a target position corresponding to the selected range thereby to switchover a shift range to the selected range.
According to JP-A-2004-23890 (US 2003/0222617), when a motor is requested to reverse its direction of rotation in response to a change of a target position during rotation of the motor, motor stop-and-hold processing is executed for a predetermined time period and then the motor is driven again to the requested target position.
The motor cannot be instantly stopped or rotated in reverse instantly while it is being rotationally driven. For this reason, when the selected range is changed by the shift operation of the driver while the motor is being rotationally driven, that is, during a switchover operation of the shift range, and the target position is changed responsively at that time, the motor may not be stopped at the target position.
According to the patent document, when the direction of rotation of the motor need be reversed in response to the change of the target position during driving of the motor, the motor stop-and-hold processing is executed for the predetermined time period and thereafter the motor is driven again to rotate to the post-change target position. However, even when the target position is changed during driving of the motor and the direction of rotation of the motor need not be reversed, the motor may not be stopped at the post-change target position in a case that the post-change target position differs only slightly from the pre-change target position which corresponds to the pre-change selected range.
It is proposed therefore that, when the selected range is changed during driving of the motor (during shift range switchover), the motor is rotationally driven to the target position corresponding to the post-change selected range to attain the switchover to the post-change selected range after the motor has been driven to the target position corresponding to the pre-change selected range and the shift range has been switched over to the pre-change selected range.
Under this control, when the selected range is changed during driving of the motor, the range is switched over to the post-change selected range after each time waiting for a completion of the switchover to the pre-change selected range. As a result, since the switchover to the post-change selected range takes more time, the response characteristic of the range switchover lowers and shift feeling of the driver is degraded.